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CRUSHING AND GRINDING                               207
material positively to the interior of the tube. At the discharge end there may be a simple, plain opening through the trunnions, or a helix may be incorporated to facilitate discharge or the discharge may be by means of radial blades. This form of discharge is accomplished by placing a steel grating about 6 in. from the discharge end of the. mill, the intervening space being occupied by a series of shelves or blades, extending radially from the periphery to near the trunnion opening. The blades deliver the ground material upon a cone whose base is against the grating, and apex extending into the trunnion opening, thus facilitating the discharge of the product. The advantages are that the discharge from a tube equipped with this system is essentially the same as a mill equipped with a peripheral discharge, but without its mechanical disadvantages. The finely ground material migrates rapidly toward the shell and discharge end of the tube, choking or plugging of the charge is avoided, and, in wet-grinding mills, it is possible to carry a low pulp level and low percentage of moisture. Crushing is more efficient, because each ball in falling, hits upon other balls covered with the material to be ground, and the blow is unimpeded. With the open-trunnion discharge, crushing is necessarily performed through a bed of pulp, which cushions the blow and dissipates the energy of the balls.
The proportions of ball mills, that is the diameter and length dimensions, have much to do with the results obtained from such mills. It is possible to so operate them that coarse, intermediate or fine product may be obtained. In this feature is one of the most favorable factors of the ball mill—it is very flexible and very adaptable.
The tonnage which may be put through a ball mill, and the fineness to which it will grind, depend on several factors, among which may be mentioned :
1.  Hardness of the ore and size of feed.
2.  Rate of travel through the mill.
3.  Percentage of water.
4.  Size of balls and number of balls.
5.  Speed of mill.
6.  Open- or closed-circuit grinding.
7.  Type of mill, center or peripheral discharge.
8.  Size of mill.
Some of these are variable or under the control of the operator; others are fixed, once the mill is running. Maximum mill efficiency is obtained when the greatest number of tons of ore is ground to the desired fineness in the shortest time, with the expenditure of a minimum amount of power and with the least wear on balls and liners. It is determined largely by the right combination of the factors above listed.
From the many excellent articles which have been written on current ball-mill practice, it is possible to gain information which will give a good idea of the correct combination necessary for the economic operation of proposed mills. No hard and fast rules can be formed, but, in general, the information gained will also prove useful in operating plants as giving a means of checking the work done by any particular mill, and of correcting existing faults.
1.  Hardness of Ore.—This is beyond the control of the operator, but its size is not. Formerly, the tendency was to feed ore up to 4 in. in size.    The more recent practice is to reduce the size of feed, for operators are determining the economic limitations of ball mills, and finding it preferable first to crush in secondary machines of approved type to 1 in. or finer.
2.  Rate of Travel—If the rate be slow, the ore is subjected to a greater number of blows, and is, of course, crushed finer.    The tendency is, however, to feed fast and return the oversize, after classifying, for regrinding (see 6).
3.  Percentage of Moisture.—This will vary with the character of the ore.    Good practice is to keep the moisture low and the pulp thick, so as to coat the balls.    This is conducive to less wear on balls and liners.